Edit: Updated to destroy KeyNav if grid is destroyed
Another Edit: a 3.x version can be found here: http://extjs.com/forum/showthread.php?p=322795
As mentioned in my presentation at the Ext Conference, here's the simple Plugin to add keyboard navigation to GridPanels with a paging toolbar. This version will currently only work with 2.x, I will be updating the code to 3.x this week and will post it once complete.

/**
* @class Ext.ux.GridKeyNav
* Simple plugin to implement basic keyboard navigation for Ext 2.x grids that have a
* paging toolbar (required). Adds: Page Down/Page Up/Right Arrow/Left Arrow/Home/End,
* also adds functionality to the selection model to see if Down is pressed at the last record
* of a page or Up is pressed at the first record of the page and attempts to move a page
* in the proper direction.
*/
Ext.ux.GridKeyNav = function() {}
Ext.ux.GridKeyNav.prototype = {
/**
* @cfg {Boolean} bottomBar
* Look for the paging toolbar within the grids bbar, set to true to pull from tbar (defaults to true)
*/
bottomBar: true,
/**
* @cfg {Ext.PagingToolbar} toolbar
* Manually pass in a toolbar instead of pulling from bbar/tbar of the grid (in case it is rendered elsewhere)
*/
toolbar: null,